This invention relates to novel hydrophilic biopolymeric materials and, more particularly, to the use of such materials in the form of hydrogel membranes as biodegradable dressings for denuded tissue wound sites such as burn wounds and ulcerations.
In the therapeutic procedure for treating an extensively burned patient, devitalized tissue is removed from the burn site, and the debrided areas are covered with a temporary burn wound dressing prior to definitive autografting. The temporary burn wound dressing ideally provides several important therapeutic functions. First of all, it serves as a barrier to prevent loss of water, salts, and proteins from the internal milieu while blocking microbial infection from the environment. Secondly, it serves to improve wound bed base and promote wound closure, thereby facilitating decontamination and regeneration of the wound area. Thirdly, it serves to alleviate pain.
The best wound coverage material is skin itself--a biologic dressing with a collagenous component rendering it adherent to endogenous fibrin, and a keratinized water proof surface. Biologic dressings in current use include commercially available pigskin heterograft, and living (donor) human or human cadaver homograft material. While human skin has a number of advantages over pigskin for this purpose, there are many problems associated with obtaining, storing, and using frozen and lyophilized human skin. Skin banks require at least 200 cadavers per year in order to supply existing burn centers. In any case, either of these types of biologic dressings give rise to rejection phenomena which mandate their removal and replacement every 2 to 5 days, or every 2 to 3 weeks if measures are taken to retard the rejection phenomenon. Eventually, however, these dressings must be stripped, often resulting in bleeding and renewed destruction of the graft base.
The body's rejection of foreign biologic dressings has lead to a search for skin substitutes which are either completely synthetic, derived from tissue components, or some combination thereof. Such search has not heretofore met with great success due to the difficulties involved in finding a material exhibiting the proper combination of properties essential for an ideal skin substitute. These properties include rapid, uniform, and strong adherence of underlying tissues; water vapor transport characteristics sufficient to keep the underlying tissues moist without creating pooling; elasticity; durability; intact bacterial barrier characteristics; nonantigenicity and nontoxicity; high permeability to oxygen; capability of being easily applied and removed; easily storable; and relatively inexpensive.
The materials previously proposed as skin substitutes have generally been found to be lacking in one or more of the foregoing properties. The most satisfactory of these materials have consisted of layered composite membranes having an outer layer designed for durability and elasticity, such as silicone or other synthetic polymeric film; and an inner layer designed for maximum adherence, such as collagen, cotton gauze, or Dacron flocking. However, the necessity for these composite membrane burn wound dressings to be stripped from the wound prior to definitive autografting, poses some difficult design problems. Since these dressings generally depend upon tissue ingrowth into their inner layer for adherence to the wound, complete removal of the dressing is difficult and may require redebridement before grafting. While it may be possible to overcome this problem by having the inner layer formed of a biodegradable material, such approach has been found to result either in a premature loss of adherence and effectiveness of the dressing before completion of wound healing, or in the formation of considerable scar tissue, unless the biodegradation rate of the inner layer is carefully controlled so as to precisely coincide with the rate of wound healing.
Homogeneous hydrogel membranes formed from the hydrophilic synthetic polymeric material, Hydron, have also been proposed as skin substitutes for use as burn wound dressings. This material combines adherency to dry and moist tissue with the other desirable properties of conformability to a regular contour, elasticity, nonantigenicity, being inert, and providing an effective antimicrobial barrier. Its major drawbacks as a burn wound dressing, however, are its excessive permeability to water and its low degree of durability when washed with water or in the presence of moderate oozing or bleeding. Moreover, its elasticity is too rapidly lost upon drying. For these reasons, it has not proven to be practical for relatively long-term burn wound dressing applications.